I have been cheated on by my ex wife, we ended our relationship hense the ex part. We grew up together and thought we were in love but really only loved certain things about each other. I believe she did this to try and find an easier way out of our relationship, I don't agree with what she had done but she has to bare that cross for the rest of her life.
If you love someone you would never hurt them and would expect the same in return, I guess the real question is, why did it happen?